Demagnifying gravitational lenses as probes of dark matter structures and nonminimal couplings to gravity

Abstract

Magnification of total image fluxes is typically considered a defining feature of gravitational microlensing. In contrast, I will show that nonminimal couplings to gravity can generate regions of negative gravitational potential curvature, giving rise to the distinctive possibility of demagnification. Such events, appearing as flux troughs in microlensing light curves, provide a direct probe of dark matter structures and, crucially, offer a means to disentangle nonminimal couplings to gravity from other astrophysical and cosmological models.

Figures (4)

• Figure 1: A typical gravitational lensing system.
• Figure 2: Threshold impact parameter $u_\mathrm{T}$ for the NFW profile, defined in equation \ref{['uT']}, for different NMC strengths. It represents the angular separation between the source and the lens below which the total magnification is reduced by at least $10\%$. Here the NMC is taken with sign $\epsilon=-1$, allowing regions of negative effective density $\rho_L$ to form.
• Figure 3: Threshold impact parameter $u_\mathrm{T}$ for the Burkert profile for different NMC strengths, using the same notation as in figure \ref{['fig:utnfw']}.
• Figure 4: Light curves for microlensing by NFW (middle) and Burkert (right) lenses, with source trajectories shown on the lens plane (left). Dashed lines in the left panel indicate trajectories with different minimum angular separations $u_0$, from 0 (blue) to 0.5 (yellow), while the black circle marks the Einstein ring. In the middle and right panels, magnification troughs appear near $\tau = 0$ for small $u_0$ in both the NFW and Burkert profiles, illustrating the demagnifying effect caused by NMCs. Here, parameters are set to $\epsilon = -1$, $L = 0.6 r_\mathrm{s}$, and $R_\mathrm{E}/r_\mathrm{s} = 3$.